Sialidases are a family of glycohydrolytic enzymes which cleave sialic acid residues from the oligosaccharide components of glycoproteins and glycolipids. Viral and bacterial enzymes have been studied, for example the influenza sialidases in particular (Air, G. M. and Laver, W. G. (1989) Proteins: Struct. Func. Genet., 6:341), but mammalian sialidases have not been well characterized. For the most part, studies of mammalian sialidases have been confined to investigation of substrate specificities and kinetic analysis using partially purified preparations, although a sialidase from rat liver and muscle has been purified to homogeneity (Miyagi, T. and Tsuiki, S. (1985) J. Biol. Chem., 260:6710). Sialidases have been identified in a number of cellular organelles: the plasma membrane (Schengrund, C., Rosenberg, A., and Repman, M. A. (1976) J. Biol. Chem., 79:555), the lysosomes and the cytosol (Tulsiani, D. R. P., and Carubelli, R., (1970) J. Biol. Chem., 245:1821).
Glycoproteins are often produced by expression of encoding genes in recombinant host cells in vitro, the cells having the normal enzyme components of cellular glycosylation machinery. Sialic acid in the oligosaccharide component of a glycoprotein is involved in mediation of clearance from the serum and affects the physical, chemical and immunogenic properties of the protein molecule. It is therefore important to maintain the sialic acid content of glycoproteins, particularly of those proteins intended for use as therapeutics.